ATOMIC STRUCTURE
 

Dalton and his contemporaries considered the atom as the smallest indivisible particle of matter. However, the concept of indivisibility was proved to be incorrect by many experiments and observed facts.
3.1 Constituents of Atom
The atom is further made of smaller particles named as subatomic or ultimate particles. The atom has been split by physicists mainly into three subatomic particles namely protons, neutrons and electrons (Table 1).

Table 1 Fundamental Units of Atom
Particle	Relative wt.	Electric charge	Atomic location	Symbol
Proton	1.0073	1	nucleus	+ or p
Neutron	1.0087	0	nucleus	n
Electron	0.0005	-1	orbital	- or e-

A proton carries a positive charge. It is placed in the nucleus of an atom

and has mass nearly equal to 1 a.m.u.
A neutron is chargeless. It is placed in the nucleus of an atom and has mass also nearly equal to 1 a.m.u.
An electron is negatively charged. It is placed in the orbital and has negligible mass.

Let’s examine the nuclear and electron structure of some of the elements (Table 2)

Table 2 Nuclear and Electron structure of some elements
Atomic number	Element	Atomic weight	Protons	Neutrons	Electrons in neutral atoms
1	Hydrogen	1	1	0	1
2	Helium	4	2	2	2
3	Lithium	7	3	4	3
4	Berylium	9	4	5	4
5	Boron	11	5	6	5
6	Carbon	12	6	6	6
7	Nitrogen	14	7	7	7
8	Oxygen	16	8	8	8
9	Fluorine	19	9	10	9
10	Neon	20	10	10	10

Protons and Neutrons are present in the nucleus of an atom whereas Electrons are placed in 4 different orbitals (s, p, d, f)) surrounding the nucleus. Atomic weight or mass number is obtained by adding the number of protons and neutrons (collectively called nucleons) in each atom. The total number of protons in the nucleus of an atom of an element is known ae the atomic number of that element. The atomic number also coincides with the number of planetary electrons in a neutral atom.
Rutherford’s atomic model is represented in the following figure:



Atomic Weights of An Elements
The atomic weight of an element is defined as a number which expresses the ratio of weight of one atom of an element to the weight of one atom of hydrogen.

For e.g. The atomic weight of nitrogen is 14 which implies that an atom of nitrogen is 14 times heavier than one atom of hydrogen.
In recent times, the naturally occurring isotope of carbon C₁₂ is taken as standard.

Thus atomic weight is only a number. It has no units and it does not provide any information regarding the actual weight of an atom of an element.
Fractional Atomic Weight :
A large number of naturally occurring elements consists of a mixture of isotopes (atoms of the same elements with the same atomic number but different atomic masses). The properties of these isotopes are constant and hence an element has a fixed atomic weight. The atomic weight of an element thus represents the average of atomic masses of different isotopes of the element. This in some cases leads to fractional atomic weight.
For e.g. Chlorine possesses two isotopes with atomic masses 35 & 37 in the proportion of 3 : 1.
Hence,

                                             = 35.5

Gram atomic weight :
The atomic weight of an element expressed in grams is known as gram atomic weight or 1 gm. atom of an element.
e.g. 1 gm. atom of Bromine = 79.9 gms. of Bromine
1 gm. atom of Sodium = 22.98 gms of Sodium
At a glance